Biology Reference
In-Depth Information
FIGURE 7.4
(Color Ýgure follows p. 206.)
Ascomata (teleomorph) of
Ophiostoma montium
in parental
galleries of
Dendroctonus ponderosae
.
J. Steimel, and T.D. Paine, unpublished observations). The most common mycangial associates of
D. adjunctus
and
D. approximatus
are
Leptographium
species with no known sexual states (Six
and Paine, 1996 and 1998). Furthermore,
spp. associated with bark beetles are
not known to produce sexual states (Goldhammer et al., 1989; Hsiau, 1996). The lack or rarity of
sexual recombination in some bark beetleÏassociated fungi may be an adaptation to mutualism. If
a fungus possesses a genotype that confers high Ýtness to both itself and to the host beetle, sexual
reproduction may be disadvantageous by continually breaking up successful gene combinations
(Wulff, 1985).
Production and dispersal of sexual and asexual spores by fungi associated with pit mycangia
appear variable.
Entomocorticium
in pit
mycangia (Furniss et al., 1990, 1995; Lewinsohn, 1994), indicating that this fungus produces
ascomata in the hostÔs pupal chambers.
D. pseudotsugae
carries both conidia and ascospores of
O. pseudotsugae
O. ips
is known to produce ascomata in pupal chambers
of
where the beetle acquires both ascospores and conidia in pit mycangia prior to emergence
(Furniss et al., 1995). Spore types have not been determined for fungi associated with beetles
possessing setal-brush mycangia (Furniss et al., 1987).
I. pini
BARK BEETLE-FUNGUS SYMBIOSES
Many scolytidÏfungus symbioses are considered mutualisms; however, many other types of asso-
ciations also exist, including antagonism, predation, and commensalism. Some saprophage associ-
ations may be commensal if the beetles beneÝt from the presence of a fungus without adversely
affecting the fungus, or antagonistic if the fungus or effects of the fungus on the wood substrate
are detrimental to the beetle, or they may involve predation if the beetle beneÝts by feeding on
fungal tissues but does not disseminate or otherwise beneÝt the fungus in return.
In contrast to saprophagous beetles, mutualism is probably widespread, although not universal,
in other scolytid groups. As with most mutualistic associations among insects and microorganisms,
beneÝts gained by either host or symbiont in these associations can be placed into three categories:
nutrition, transport, and direct protection.
N
UTRITION
Nutrition is the major driving force in mycetophage scolytid associations (Norris and Baker, 1967;
Norris et al., 1969; Abrahamson and Norris, 1970; Kok, 1979) and in many other obligate insectÏfun-
gal symbioses (Wetzel et al., 1992). It is also likely to be an important factor driving many associations
of fungi with phytophage bark beetles. Bark beetle larvae feed on mycelial fungi as larvae and teneral
